The government's denial came as a bomb reportedly ripped through a bus in the main market of Dera Bugti in Baluchistan province.

"There was a bomb explosion in the main bazaar and seven people were killed and six are wounded," Hazoor Baksh, a local police officer, told the AFP news agency.

No one immediately claimed responsibility for the attack, but the region has seen sporadic violence by ethnic Baluch tribesmen demanding greater political autonomy and a larger share of profits from the province's plentiful natural resources.

There have been fears after Thursday's assassination attempt on Bhutto that further attacks could damage Pakistan's already fractured political landscape.

Political atmosphere

In an editorial titled "Monster of Extremism,", The News, an independent national newspaper, said on Saturday that the bombings "could further polarise an already charged political atmosphere in the country and bring yet more uncertainty to the country's fledgling democratic system".

Azhar Farooqi, Karachi police chief, told the AFP news agency: "Investigations are progressing in the right direction, but once again we will avoid pinpointing anyone or blaming it on a certain set of militants.

"There has been a minor piece of evidence found from the site today but we will not be disclosing it. Police have secured the site of the blast and taken samples."

Several hundred protesters burned tyres in pro-Bhutto neighbourhoods of Karachi on Saturday and pelted passing vehicles with stones for a second day, witnesses said.

"They are expressing their anguish over the attack on their leader and the deaths of so many people," Fayaz Khan, a senior police officer, said. There were no immediate reports of injuries.

Bhutto has pledged to stay in Pakistan to fight general elections scheduled for January, but the attack on her motorcade, the worst suicide bombing in Pakistan's history, has cast doubt over her plans to tour the country to gather support ahead of the polls.

Her bulletproof bus, thronged by supporters, had been moving towards the centre of Karachi for 10 hours when a small explosion was heard near the front of the vehicle. That was quickly followed by a larger blast, destroying two police vans that were escorting it.